Although a mobile telephone is small in size, it has a loudspeaker for ringing or amplifying the sound in addition to a receiving loudspeaker. In particular, since there is a limit to miniaturization of small-sized equipment such as folding type equipment, two loudspeaker parts are built in one frame. FIG. 8 is a sectional view showing a conventional loudspeaker of this kind.
Hollow cylindrical frame 1 formed by resin molding has convex part 1A protruding inwardly all around a center part of an inner circumferential surface. Ring-shaped first magnet 2 is bonded to convex portion 1A at its outer circumferential side surface. Cup-shaped yoke 3 made of a magnetic material such as iron is bonded to first magnet 2 at its lower surface of an outer circumference. Columnar second magnet 4 is bonded to a center part of yoke 3. Annular first plate 6 is bonded to a lower surface of first magnet 2, and second plate 5 is bonded to an upper surface of second magnet 4. Annular first magnetic gap 7 is provided between an inner circumference of first plate 6 and the outer circumference of the center part of yoke 3. Annular second magnetic gap 8 is formed between an outer circumference of second plate 5 and an inner circumference of the center part of yoke 3.
In the above-mentioned configuration, first magnet 2, yoke 3, first magnetic gap 7 and first plate 6 form a first magnetic circuit. Furthermore, second magnet 4, second plate 5, second magnetic gap 8 and yoke 3 form a second magnetic circuit.
First diaphragm 9 is placed at a lower side opening of frame 1. Annular first voice coil 10 is bonded to first diaphragm 9 at its lower end and located in first magnetic gap 7 at another end. First protector 10A has a plurality of holes for releasing sound and is bonded to outer circumferences of frame 1 and first diaphragm 9 such that it covers first diaphragm 9. Second diaphragm 11 is bonded to an upper side opening of frame 1. Annular second voice coil 12 is bonded to second diaphragm 11 at its upper end and located in second magnetic gap 8 at another end. Second protector 13 has a plurality of holes for releasing sound and is bonded to outer circumferences of frame 1 and second diaphragm 11 such that it covers second diaphragm 11. Such a loudspeaker is disclosed in, for example, Japanese Patent Unexamined Publication No. 2003-111194.
When a loudspeaker having the above-mentioned configuration including two loudspeaker parts is used for, for example, a mobile telephone, one loudspeaker part is used as a receiver for receiving and another is used for a loudspeaker for notifying incoming of call or a loudspeaker. Furthermore, the loudspeaker can be used as a small-sized stereo loudspeaker by inputting LR signals, respectively.
However, since in the loudspeaker having such a configuration, two magnetic circuits are built in, it is difficult to reduce weight. Also, the configuration is complicated, resulting in increase in the number of components and the man-hour required for assembly. Accordingly, the cost is high. Note here that although there is a loudspeaker having two diaphragms in one magnetic circuit, variation is likely to occur in a sound pressure frequency property. Therefore, strict assembly accuracy is required, and assembly itself is complex